Cholesteric liquid crystalline indicator compositions are well-known in the art as optical indicators of transient environmental conditions. These compositions typically depend for their effect on a change in optical properties in response to a change in environmental conditions, such as a shift from color-exhibiting to colorless in response to a change in temperature.
Several difficulties have been encountered in the use of cholesteric substances as thermometric compositions, primarily owing to the rapid reversibility of the changes in optical properties. In many applications, it is highly desirable to have a permanent record that a predetermined temperature on other environmental condition has occurred, for example in the storage of food or sensitive biological materials. Many cholesteric substances, however, provide only fleeting records of fluctuating environmental conditions, rapidly changing color or gaining or losing color in response to stimulus. While methods have been devised to prolong these optical changes in some liquid crystal systems, the record of the occurrence of a predetermined condition generally is still too evanescent to be useful in applications where regular monitoring is not feasible.
It is accordingly an object of this invention to provide a method and means for irreversibly recording an environmental event.
It is another object of this invention to provide a method and means for making a permanent visual record of the occurrence of a predetermined temperature.
It is a further object of this invention to provide an indicator system for selectively indicating the occurrence of a predetermined temperature over a wide range of temperatures.
It is yet another object of this invention to provide a rapid and permanent indication of the occurrence of a predetermined temperature which is readily apparent to the naked eye.
It is also an object of this invention to provide a system and method for irreversibly recording the occurrence of a predetermined maximum temperature.
Other objects and advantages of the invention will be readily apparent to those skilled in the art.